module add_three_numbers
(
input clock,
input [7:0] A,
input [7:0] B,
input [7:0] C,
input [7:0] D,
output [7:0] sum
);

reg [7:0] reg_A, reg_B, reg_C, reg_D;
reg [7:0] reg_sum;

reg [7:0] sum1, sum2;
reg [7:0] sum3;


always@(posedge clock)begin
reg_A <= A;
reg_B <= B;
reg_C <= C;
reg_D <= D;
end

always@(posedge clock)begin
sum1 <= reg_A + reg_B;
sum2 <= reg_C + reg_D;
sum3 <= sum1 + sum2;
end


always@(posedge clock)begin
reg_sum <= sum3;
end

assign sum = reg_sum;

endmodule

create_clock -name {clock} -period 400Mhz [get_ports {clock}]